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+//===- PoisonChecking.cpp - -----------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Implements a transform pass which instruments IR such that poison semantics
+// are made explicit. That is, it provides a (possibly partial) executable
+// semantics for every instruction w.r.t. poison as specified in the LLVM
+// LangRef. There are obvious parallels to the sanitizer tools, but this pass
+// is focused purely on the semantics of LLVM IR, not any particular source
+// language. If you're looking for something to see if your C/C++ contains
+// UB, this is not it.
+//
+// The rewritten semantics of each instruction will include the following
+// components:
+//
+// 1) The original instruction, unmodified.
+// 2) A propagation rule which translates dynamic information about the poison
+// state of each input to whether the dynamic output of the instruction
+// produces poison.
+// 3) A flag validation rule which validates any poison producing flags on the
+// instruction itself (e.g. checks for overflow on nsw).
+// 4) A check rule which traps (to a handler function) if this instruction must
+// execute undefined behavior given the poison state of it's inputs.
+//
+// At the moment, the UB detection is done in a best effort manner; that is,
+// the resulting code may produce a false negative result (not report UB when
+// it actually exists according to the LangRef spec), but should never produce
+// a false positive (report UB where it doesn't exist). The intention is to
+// eventually support a "strict" mode which never dynamically reports a false
+// negative at the cost of rejecting some valid inputs to translation.
+//
+// Use cases for this pass include:
+// - Understanding (and testing!) the implications of the definition of poison
+// from the LangRef.
+// - Validating the output of a IR fuzzer to ensure that all programs produced
+// are well defined on the specific input used.
+// - Finding/confirming poison specific miscompiles by checking the poison
+// status of an input/IR pair is the same before and after an optimization
+// transform.
+// - Checking that a bugpoint reduction does not introduce UB which didn't
+// exist in the original program being reduced.
+//
+// The major sources of inaccuracy are currently:
+// - Most validation rules not yet implemented for instructions with poison
+// relavant flags. At the moment, only nsw/nuw on add/sub are supported.
+// - UB which is control dependent on a branch on poison is not yet
+// reported. Currently, only data flow dependence is modeled.
+// - Poison which is propagated through memory is not modeled. As such,
+// storing poison to memory and then reloading it will cause a false negative
+// as we consider the reloaded value to not be poisoned.
+// - Poison propagation across function boundaries is not modeled. At the
+// moment, all arguments and return values are assumed not to be poison.
+// - Undef is not modeled. In particular, the optimizer's freedom to pick
+// concrete values for undef bits so as to maximize potential for producing
+// poison is not modeled.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Instrumentation/PoisonChecking.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/MemoryBuiltins.h"
+#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/IR/InstVisitor.h"
+#include "llvm/IR/IntrinsicInst.h"
+#include "llvm/IR/IRBuilder.h"
+#include "llvm/IR/PatternMatch.h"
+#include "llvm/Support/Debug.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "poison-checking"
+
+static cl::opt<bool>
+LocalCheck("poison-checking-function-local",
+ cl::init(false),
+ cl::desc("Check that returns are non-poison (for testing)"));
+
+
+static bool isConstantFalse(Value* V) {
+ assert(V->getType()->isIntegerTy(1));
+ if (auto *CI = dyn_cast<ConstantInt>(V))
+ return CI->isZero();
+ return false;
+}
+
+static Value *buildOrChain(IRBuilder<> &B, ArrayRef<Value*> Ops) {
+ if (Ops.size() == 0)
+ return B.getFalse();
+ unsigned i = 0;
+ for (; i < Ops.size() && isConstantFalse(Ops[i]); i++) {}
+ if (i == Ops.size())
+ return B.getFalse();
+ Value *Accum = Ops[i++];
+ for (; i < Ops.size(); i++)
+ if (!isConstantFalse(Ops[i]))
+ Accum = B.CreateOr(Accum, Ops[i]);
+ return Accum;
+}
+
+static void generatePoisonChecksForBinOp(Instruction &I,
+ SmallVector<Value*, 2> &Checks) {
+ assert(isa<BinaryOperator>(I));
+
+ IRBuilder<> B(&I);
+ Value *LHS = I.getOperand(0);
+ Value *RHS = I.getOperand(1);
+ switch (I.getOpcode()) {
+ default:
+ return;
+ case Instruction::Add: {
+ if (I.hasNoSignedWrap()) {
+ auto *OverflowOp =
+ B.CreateBinaryIntrinsic(Intrinsic::sadd_with_overflow, LHS, RHS);
+ Checks.push_back(B.CreateExtractValue(OverflowOp, 1));
+ }
+ if (I.hasNoUnsignedWrap()) {
+ auto *OverflowOp =
+ B.CreateBinaryIntrinsic(Intrinsic::uadd_with_overflow, LHS, RHS);
+ Checks.push_back(B.CreateExtractValue(OverflowOp, 1));
+ }
+ break;
+ }
+ case Instruction::Sub: {
+ if (I.hasNoSignedWrap()) {
+ auto *OverflowOp =
+ B.CreateBinaryIntrinsic(Intrinsic::ssub_with_overflow, LHS, RHS);
+ Checks.push_back(B.CreateExtractValue(OverflowOp, 1));
+ }
+ if (I.hasNoUnsignedWrap()) {
+ auto *OverflowOp =
+ B.CreateBinaryIntrinsic(Intrinsic::usub_with_overflow, LHS, RHS);
+ Checks.push_back(B.CreateExtractValue(OverflowOp, 1));
+ }
+ break;
+ }
+ case Instruction::Mul: {
+ if (I.hasNoSignedWrap()) {
+ auto *OverflowOp =
+ B.CreateBinaryIntrinsic(Intrinsic::smul_with_overflow, LHS, RHS);
+ Checks.push_back(B.CreateExtractValue(OverflowOp, 1));
+ }
+ if (I.hasNoUnsignedWrap()) {
+ auto *OverflowOp =
+ B.CreateBinaryIntrinsic(Intrinsic::umul_with_overflow, LHS, RHS);
+ Checks.push_back(B.CreateExtractValue(OverflowOp, 1));
+ }
+ break;
+ }
+ case Instruction::UDiv: {
+ if (I.isExact()) {
+ auto *Check =
+ B.CreateICmp(ICmpInst::ICMP_NE, B.CreateURem(LHS, RHS),
+ ConstantInt::get(LHS->getType(), 0));
+ Checks.push_back(Check);
+ }
+ break;
+ }
+ case Instruction::SDiv: {
+ if (I.isExact()) {
+ auto *Check =
+ B.CreateICmp(ICmpInst::ICMP_NE, B.CreateSRem(LHS, RHS),
+ ConstantInt::get(LHS->getType(), 0));
+ Checks.push_back(Check);
+ }
+ break;
+ }
+ case Instruction::AShr:
+ case Instruction::LShr:
+ case Instruction::Shl: {
+ Value *ShiftCheck =
+ B.CreateICmp(ICmpInst::ICMP_UGE, RHS,
+ ConstantInt::get(RHS->getType(),
+ LHS->getType()->getScalarSizeInBits()));
+ Checks.push_back(ShiftCheck);
+ break;
+ }
+ };
+}
+
+static Value* generatePoisonChecks(Instruction &I) {
+ IRBuilder<> B(&I);
+ SmallVector<Value*, 2> Checks;
+ if (isa<BinaryOperator>(I) && !I.getType()->isVectorTy())
+ generatePoisonChecksForBinOp(I, Checks);
+
+ // Handle non-binops seperately
+ switch (I.getOpcode()) {
+ default:
+ break;
+ case Instruction::ExtractElement: {
+ Value *Vec = I.getOperand(0);
+ if (Vec->getType()->getVectorIsScalable())
+ break;
+ Value *Idx = I.getOperand(1);
+ unsigned NumElts = Vec->getType()->getVectorNumElements();
+ Value *Check =
+ B.CreateICmp(ICmpInst::ICMP_UGE, Idx,
+ ConstantInt::get(Idx->getType(), NumElts));
+ Checks.push_back(Check);
+ break;
+ }
+ case Instruction::InsertElement: {
+ Value *Vec = I.getOperand(0);
+ if (Vec->getType()->getVectorIsScalable())
+ break;
+ Value *Idx = I.getOperand(2);
+ unsigned NumElts = Vec->getType()->getVectorNumElements();
+ Value *Check =
+ B.CreateICmp(ICmpInst::ICMP_UGE, Idx,
+ ConstantInt::get(Idx->getType(), NumElts));
+ Checks.push_back(Check);
+ break;
+ }
+ };
+ return buildOrChain(B, Checks);
+}
+
+static Value *getPoisonFor(DenseMap<Value *, Value *> &ValToPoison, Value *V) {
+ auto Itr = ValToPoison.find(V);
+ if (Itr != ValToPoison.end())
+ return Itr->second;
+ if (isa<Constant>(V)) {
+ return ConstantInt::getFalse(V->getContext());
+ }
+ // Return false for unknwon values - this implements a non-strict mode where
+ // unhandled IR constructs are simply considered to never produce poison. At
+ // some point in the future, we probably want a "strict mode" for testing if
+ // nothing else.
+ return ConstantInt::getFalse(V->getContext());
+}
+
+static void CreateAssert(IRBuilder<> &B, Value *Cond) {
+ assert(Cond->getType()->isIntegerTy(1));
+ if (auto *CI = dyn_cast<ConstantInt>(Cond))
+ if (CI->isAllOnesValue())
+ return;
+
+ Module *M = B.GetInsertBlock()->getModule();
+ M->getOrInsertFunction("__poison_checker_assert",
+ Type::getVoidTy(M->getContext()),
+ Type::getInt1Ty(M->getContext()));
+ Function *TrapFunc = M->getFunction("__poison_checker_assert");
+ B.CreateCall(TrapFunc, Cond);
+}
+
+static void CreateAssertNot(IRBuilder<> &B, Value *Cond) {
+ assert(Cond->getType()->isIntegerTy(1));
+ CreateAssert(B, B.CreateNot(Cond));
+}
+
+static bool rewrite(Function &F) {
+ auto * const Int1Ty = Type::getInt1Ty(F.getContext());
+
+ DenseMap<Value *, Value *> ValToPoison;
+
+ for (BasicBlock &BB : F)
+ for (auto I = BB.begin(); isa<PHINode>(&*I); I++) {
+ auto *OldPHI = cast<PHINode>(&*I);
+ auto *NewPHI = PHINode::Create(Int1Ty,
+ OldPHI->getNumIncomingValues());
+ for (unsigned i = 0; i < OldPHI->getNumIncomingValues(); i++)
+ NewPHI->addIncoming(UndefValue::get(Int1Ty),
+ OldPHI->getIncomingBlock(i));
+ NewPHI->insertBefore(OldPHI);
+ ValToPoison[OldPHI] = NewPHI;
+ }
+
+ for (BasicBlock &BB : F)
+ for (Instruction &I : BB) {
+ if (isa<PHINode>(I)) continue;
+
+ IRBuilder<> B(cast<Instruction>(&I));
+
+ // Note: There are many more sources of documented UB, but this pass only
+ // attempts to find UB triggered by propagation of poison.
+ if (Value *Op = const_cast<Value*>(getGuaranteedNonFullPoisonOp(&I)))
+ CreateAssertNot(B, getPoisonFor(ValToPoison, Op));
+
+ if (LocalCheck)
+ if (auto *RI = dyn_cast<ReturnInst>(&I))
+ if (RI->getNumOperands() != 0) {
+ Value *Op = RI->getOperand(0);
+ CreateAssertNot(B, getPoisonFor(ValToPoison, Op));
+ }
+
+ SmallVector<Value*, 4> Checks;
+ if (propagatesFullPoison(&I))
+ for (Value *V : I.operands())
+ Checks.push_back(getPoisonFor(ValToPoison, V));
+
+ if (auto *Check = generatePoisonChecks(I))
+ Checks.push_back(Check);
+ ValToPoison[&I] = buildOrChain(B, Checks);
+ }
+
+ for (BasicBlock &BB : F)
+ for (auto I = BB.begin(); isa<PHINode>(&*I); I++) {
+ auto *OldPHI = cast<PHINode>(&*I);
+ if (!ValToPoison.count(OldPHI))
+ continue; // skip the newly inserted phis
+ auto *NewPHI = cast<PHINode>(ValToPoison[OldPHI]);
+ for (unsigned i = 0; i < OldPHI->getNumIncomingValues(); i++) {
+ auto *OldVal = OldPHI->getIncomingValue(i);
+ NewPHI->setIncomingValue(i, getPoisonFor(ValToPoison, OldVal));
+ }
+ }
+ return true;
+}
+
+
+PreservedAnalyses PoisonCheckingPass::run(Module &M,
+ ModuleAnalysisManager &AM) {
+ bool Changed = false;
+ for (auto &F : M)
+ Changed |= rewrite(F);
+
+ return Changed ? PreservedAnalyses::none() : PreservedAnalyses::all();
+}
+
+PreservedAnalyses PoisonCheckingPass::run(Function &F,
+ FunctionAnalysisManager &AM) {
+ return rewrite(F) ? PreservedAnalyses::none() : PreservedAnalyses::all();
+}
+
+
+/* Major TODO Items:
+ - Control dependent poison UB
+ - Strict mode - (i.e. must analyze every operand)
+ - Poison through memory
+ - Function ABIs
+ - Full coverage of intrinsics, etc.. (ouch)
+
+ Instructions w/Unclear Semantics:
+ - shufflevector - It would seem reasonable for an out of bounds mask element
+ to produce poison, but the LangRef does not state.
+ - and/or - It would seem reasonable for poison to propagate from both
+ arguments, but LangRef doesn't state and propagatesFullPoison doesn't
+ include these two.
+ - all binary ops w/vector operands - The likely interpretation would be that
+ any element overflowing should produce poison for the entire result, but
+ the LangRef does not state.
+ - Floating point binary ops w/fmf flags other than (nnan, noinfs). It seems
+ strange that only certian flags should be documented as producing poison.
+
+ Cases of clear poison semantics not yet implemented:
+ - Exact flags on ashr/lshr produce poison
+ - NSW/NUW flags on shl produce poison
+ - Inbounds flag on getelementptr produce poison
+ - fptosi/fptoui (out of bounds input) produce poison
+ - Scalable vector types for insertelement/extractelement
+ - Floating point binary ops w/fmf nnan/noinfs flags produce poison
+ */
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